Microporous Lithium-ion battery separator film, “BSF”, is typically polypropylene or polyethylene film between approximately 8-40 μm thickness, 5-25 g/m2 mass per unit area, 0.5-0.65 g/cm3 density.
Micropores in the film allow the propagation of charged ions between the cathode and anode of the battery. These micropores may not be uniformly distributed in the film. Therefore a film with uniform mass per unit area may still have significant variations in its density (and therefore thickness), depending on the distribution of micropores in the film.
Producers of BSF wish to know the variation of thickness, mass per unit area, density and/or porosity of film they are making across their production web, for quality assurance and production control purposes. The standard method for measuring the mass per unit area of polymer films on a moving web by infrared, X-ray, gamma ray or beta particles is to measure the transmitted radiation through the film and compare this to a standardised reading, taken with no film present. The heavier the film the less radiation is transmitted. The thickness of the film is then interpreted from the mass per unit area, by assuming that the film has a constant, uniform density. This causes unacceptable errors for any films that do not have uniform density across.
BSF does not have a uniform density because of micropores. Therefore, only an approximation of the thickness can be obtained using this method, rather than the true thickness. The more non-uniform the density of the film, the worse the approximation will be. It is also impossible to measure the density of the film using this method.
When infrared radiation interacts with microporous polymer film, some wavelengths are molecularly absorbed by the film. The wavelengths which are absorbed depend on the polymer. For example, polyethylene exhibits absorption centres around 2315, 2350 nm and between 3300-3600 nm. The snore polymer that is present, the more radiation is absorbed at these wavelengths. Therefore by measuring the amount of radiation transmuted through the film at the absorption wavelengths, information on the mass (mass per unit area) of the film can be ascertained.
Infrared radiation is also elastically scattered by the micropores in the film; the shorter the wavelength, the greater degree of scattering from the micropores. Short wavelengths (1500-2500 nm) are scattered more than longer ones (2500-5000 nm). The amount of scatter also depends on the number of micropores per unit volume (and hence the density of the film) and their morphology (size and shape). Porosity is a function of micropores quantity, size, and shape.
Scattering effects are therefore apparent as a continuous shift in the baseline in the near-infrared/mid-infrared, “NIR-MIR”, spectrum, with the transmission of infrared, “IR”, radiation directly through the BSF decreasing with shorter wavelengths. Low density BSF films have more micropores per unit volume and hence scatter IR to a greater degree. Therefore the lower the density the more pronounced the baseline change with shorter wavelengths. The resulting directly transmitted spectrum is therefore a combination of a continuous baseline variation, (which is a function of wavelength, film density and morphology of micropores in the film) and absorbance features (dependant on polymer type and mass per unit area of the film).
FIG. 1 shows example transmission spectra of polymer film with different scattering characteristics, but the same mass per unit area.
FIG. 1 shows three NIR-MIR transmission spectra: a first spectrum 101, a second spectrum 103 and a third spectrum 105. The first spectrum 101 is the NIR-MIR transmission spectrum obtained from a clear polymer film with no micropores. A second spectrum 103 is the NIR-MIR transmission spectra obtained from a second polymer film with micropores and therefore a lower density than the clear polymer film. A third spectrum 105 is the NIR-MIR transmission spectra obtained from a third film with a greater number of micropores and a lower density than the second polymer film. The chemical compositions of the three polymer films are identical and so the absorption peaks can be seen in the first spectrum 101, second spectrum 103 and third spectrum 105 at the same wavelengths.
A first arrow 150 indicates a first trend of increasing elastic scatter. As shown in FIG. 1, the proportion of NIR-MIR radiation scattered increases with decreasing film density. As also shown its FIG. 1, a second trend indicated by second arrow 160 also indicates a decrease in scatter with increasing wavelength. The second trend is attributed to the fact that scatter from micropores is more pronounced at shorter wavelengths in this region of the spectrum. This results in a baseline shift in the spectrum due to elastic scatter, dependant on wavelength; the slope of which depends on the number and morphology of the micropores in the film.
The current methods for measuring elastically scattering materials with NIR or MIR view baseline changes due to scattering effects as undesirable. Therefore these methods seek to remove it, so that the absorptions may be measured more effectively. This may be done by combining simultaneously measured transmission and diffuse reflectance spectra. See, for example, U.S. Pat. No. 4,602,160 which discloses a technique for measuring constituents in a moving paper web.
Alternatively, empirically-derived scattering models may be applied to the data in order to produce scatter-corrected spectra, such as multiplicative scatter correction (MSC) and extended multiplicative scatter correction (EMSC). See, for example, Journal of Anal. Chem 2003, 75, pp 394-404, and Anal. Lett. 2011, 44 pp 824-836 for an overall review.
Notably, previous methods focused on removing scattered-related features from the obtained spectra to improve measurement performance In contrast, the present disclosure describes methods for deriving useful information about, a sample from the scatter-related features which were previously discarded.